1. Field of the Invention
The present invention relates to a pattern area value calculating method, a method of calculating a proximity effect-corrected dose, a charged particle beam writing method, and a charged particle beam writing apparatus. For example, the present invention relates to a proximity effect correcting technique (to be described below) A pattern to be written is divided into predetermined unit sections (grids) The present invention relates to a proximity effect correcting technique which corrects a dose of an electron beam to be irradiated on each unit section in consideration of accumulated energy caused by back scattering of electrons.
2. Related Art
A lithography technique which leads development of micropatterning of semiconductor devices is a very important process which uniquely generates a pattern in semiconductor manufacturing processes. In recent years, with high integration of an LSI, a circuit line width required for semiconductor devices progressively decreases year after year. In order to form a desired circuit pattern on the semiconductor devices, a high-definition original pattern (also called a reticle or a mask) is necessary. In this case, an electron beam writing technique has an essentially excellent resolution and is used in production of a high-definition original pattern.
FIG. 11 is a conceptual diagram for explaining an operation of a conventional variable-shaped electron beam photolithography apparatus.
A variable-shaped electron beam photolithography apparatus (electron beam (EB) writing apparatus) operates as follows. In a first aperture 410, a square, for example, rectangular opening 411 to shape an electron beam 330 is formed. In a second aperture 420, a variable-shaped opening 421 to shape the electron beam 330 having passed through the opening 411 formed in the first aperture 410 in a desired square shape is formed. The electron beam 330 irradiated from a charged particle source 430 and having passed through the opening 411 is deflected by a deflector. The electron beam 330 passes through a part of the variable-shaped opening 421 and is irradiated on a target object 340 placed on a stage. The stage continuously moves in one predetermined direction (for example, defined as an X direction) while irradiating the electron beam 330. More specifically, a square shape which can pass through both the opening 411 and the variable-shaped opening 421 is written in a writing region of the target object 340 placed on the stage. A scheme which causes an electron beam to pass through both the opening 411 and the variable-shaped opening 421 to form an arbitrary shape is called a variable shaped scheme.
A pattern of a semiconductor integrated circuit is written on a resist material formed on the target object 340 by using an electron beam. In this case, the electron beam used in the pattern writing passes through the resist material and is incident on the target object 340. Then, back scattering occurs. A part of the electron beam is incident on the resist material again. As a result, the resist material is exposed in an area which is considerably larger than an incident part of the electron beam not to obtain a pattern having a desired line width. When patterns to be written approximate to each other due to micropatterning to increase the density, exposure of the resist material caused by back scattering occurs in a very wide range. Since this proximity effect is caused, correction must be performed. In general, when a pattern is written on the resist material on the substrate, a pattern to be written is divided into predetermined unit sections (to be referred to as grids or meshes). At the center of each unit section, accumulated energy caused by back scattering is calculated on the basis of an EID function. In consideration of the accumulated energy, a dose of an electron beam to be irradiated on each unit section is corrected.
In relation to the proximity effect correction, a technique which calculates accumulated energy on the basis of an EID function by replacing the center point of the unit section with an area gravity point is disclosed in a reference (for example, see JP-A-9-186058).
As described above, in the calculation of the proximity effect correction, a pattern to be written is divided into predetermined meshes, and accumulated energy caused by back scattering is calculated on the basis of an EID function at a center position of each mesh.
However, an area value included in a mesh is regarded to be concentrated on the center of the mesh to estimate a back scattering energy distribution. For this reason, the position is different from an arrangement position of an actual pattern. As a consequence, the back scattering energy distribution has an error. Since the back scattering energy distribution with the error is used in calculation of a beam dose on the pattern, the error adversely affects the calculation. Therefore, the beam dose to be calculated also has an error. The conventional technique has the above problems. At the present or in the future, with an increase in degree of integration density of an LSI, highly accurate proximity effect correction is required. In this circumstance, an error caused by indetermination of the area position is a factor which decreases correction accuracy near a figure.